1. Field of the Invention
This invention relates to a hydrogen storage alloy which is adapted for use as a negative electrode of alkaline batteries. More particularly, the invention relates to a hydrogen storage alloy which exhibits a small gradient in a plateau region of a PCT (pressure composition isotherm) characteristic curve, and a method for producing such an alloy. The invention also relates to a sintered product of the alloy obtained by the method.
2. Description of the Prior Art
After the discovery of hydrogen storage alloys capable of storing and releasing hydrogen, extensive studies on the application of the alloys have been made. In fact, alkaline batteries, which make use of hydrogen storage alloys as a negative electrode, have been already put into practice. Improved hydrogen storage alloys have been successively developed for use in the batteries.
LaNi.sub.5 alloy, which was initially developed (see Japanese Laid-open Patent Application No. 51-13934) is advantageous in its great storage of hydrogen. However, La is inconveniently expensive, coupled with another problem that the alloy is liable to be broken into fine pieces when repeating storage and release of hydrogen and is prone to be corroded on contact with alkaline or acid solutions. In fact, when using the hydrogen storage alloy as a negative electrode of alkaline batteries, the battery exhibits a high initial capacity, but the capacity is reduced to a half of the initial capacity when about 50 charge and discharge cycles are repeated, thus presenting the problem that the electrode cannot stand use over a long time.
In order to overcome the problem, there has been proposed the use of Misch metal (hereinafter referred to simply as Mm) wherein part of La is replaced by Ce, Pr, Nd or other rare earth metals, or a LaNi.sub.5 hydrogen storage alloy wherein part of Ni is placed by a metal such as Co, Al, Mn or the like. These proposals are set out, for example, in Japanese Laid-open Patent Application Nos. 53-4918, 54-64014, 60-250558, 61-91862 and 61-233969.
Another method has been proposed in order to overcome the above-stated problem. In the method, there is used a hydrogen storage alloy which has been broken into fine pieces to an extent that they suffer further breakage into finer pieces. Such fine alloy particles undesirably exhibit a great gradient in the plateau region of the PCT characteristic curve of the hydrogen storage alloy. In the worst case, the plateau region may disappear, making it difficult to use the alloy in practical applications.